Reckoning

Reckoning

[edit] Meteorological

Animation of seasonal differences especially snow cover through the year

Meteorological seasons are reckoned by temperature, with summer being the hottest quarter of the year and winter the coldest quarter of the year. Using this reckoning, the Roman calendar began the year and the spring season on the first of March, with each season occupying three months. In 1780 the Societas Meteorologica Palatina, an early international organization for meteorology, defined seasons as groupings of three whole months. Ever since, professional meteorologists all over the world have used this definition.^[7] Therefore, in meteorology for the Northern hemisphere, spring begins on 1 March, summer on 1 June, autumn on 1 September, and winter on 1 December.
In Sweden and Finland, meteorologists use a different definition for the seasons, based on the temperature: spring begins when the daily averaged temperature permanently rises above 0° C, summer begins when the temperature permanently rises above +10° C, summer ends when the temperature permanently falls below +10° C and winter begins when the temperature permanently falls below 0° C. "Permanently" here means that the daily averaged temperature has remained above or below the limit for seven consecutive days. This implies two things: first, the seasons do not begin at fixed dates but must be determined by observation and are known only after the fact; and second, a new season begins at different dates in different parts of the country